Furnace

ABSTRACT

A furnace provided with three heating mantles mounted around each other and separated by insulating layers, wherein the innermost, i.e., the first mantle is connected to a first supply voltage source, and the second and the third mantles are connected in series to a second supply voltage source, which second source is controlled by heat flow in the insulating layer located between the first and second mantles, the second and third mantles being constructed such that the power excitable per unit of surface in the mantles are substantially equal.

United States Patent Dijkstra et al.

FURNACE lnventors: Fokke Cornelius Dijkstra,

Maastricht; Theodorus Maria Groothuizen, Schipluizen; Hermanus JohannesPasman, Wassenaar; Tonny Van de Putte, De Lier, all of NetherlandsNederlandse Organisatie Voor Togepast-Natuurweten-Schappelijk OnderzoekTen Behoeve Van De Rijksverdediging, The Hague, Netherlands Filed: Aug.1, 1973 Appl. No.: 384,569

Assignee:

Foreign Application Priority Data [56] References Cited UNlTED STATESPATENTS 3,112,919 12/1963 Gunow 13/20 X 3,427,011 2/1969 Boyer et a1.13/31 X Primary ExaminerR. N. Envall, Jr. Attorney, Agent, orFirmWatson, Cole, Grindle & Watson 5 7 ABSTRACT A furnace provided withthree heating mantles mounted around each other and separated byinsulating layers, wherein the innermost, i.e., the first mantle isconnected to a first supply voltage source, and the second and the thirdmantles are connected in series to a second supply voltage source, whichsecond source is controlled by heat flow in the insulating layer Aug. 2,1972 Netherlands 7210578 located between the first and Second mantlesthe ond and third mantles being constructed such that the US. Cl 13/22,13/24, 13/31 power excitable per unit of surface in the mantles are Int.Cl. F27b 5/08, F27d. 1 1/02 substantially equal Field of Search 13/20,22, 25, 31;

219 209 2 0 5 39 1 Claim, 1 Drawing Figure HQ 2 we A n W v L 3 J mmcmoit4 "1O 1 1Q K 15 4 RELAY 1: CONTROLLER gggggg FURNACE The inventionrelates to a furnace, comprising a hollow metal housing for containing asample in its cavity and provided with a first electric heating mantle,a second electric heating mantle around the first mantle, aheat-insulating intermediate layer between the two mantles and means forregulating the flow of heat in the intermediate layer. Such a furnace isgenerally known in the art.

The second electric heating mantle about the first mantle, theheat-insulating intermediate layer between the two mantles and the meansfor controlling the flow of heat in the intermediate layer in this knownfurnace are provided to render the furnace adiabatic, i.e., thefrequency of the impulses of current through the second mantle iscontrolled in such a way that the flow of heat through the intermediatelayer becomes practically zero.

A slight modification in the temperature of the first mantle is thenfollowed by a slight modification in the temperature of the secondmantle. If the housing is to pass linearly through a temperaturetrajectory however, then it should be sufficient to provide the firstmantle with a constant current. In practice it has been found, however,that then the impulses of current as a result of readjusting the secondmantle are highly perceptible with respect to the temperature of themetal housing.

It is the object of the invention to improve such an arrangement.Therefore, a third electric heating mantle is mounted about the secondmantle. This third mantle is provided with an external heat-insulatinglayer, and a heat-insulating layer is disposed between these twomantles. The second and third mantles are electrically connected inseries, and the power excitable per unit of surface in the second andthe third mantles are practically equal. They are controlled as a wholeby the means for controlling the flow of heat in the intermediate layer,and the supply voltage of the first mantle is independent of the supplyvoltage control of the series connection.

With such an arrangement, the third mantle is found to be always lowerin temperature than the second mantle, and the impulses of current inthese mantles do not or hardly interfere with respect to the temperatureof the housing.

The invention will be further described with reference to the drawing,showing an embodiment of a furnace according to the invention.

In the FIGURE furnace space 1 is surrounded by a metal housing 2 and alid 20. Housing 2 is provided with a first heating mantle 3, connectedto a stabilized current source 4. ln furnace space 1 a temperature probe5 is mounted, connected to a registrating and/or indicating device 6.

A heat-insulating intermediate layer 7 is mounted between heating mantle4 and a second heating mantle 8.

A third heating mantle 9 is connected in series with mantle 8, whichmantle 8 is mounted on a heatinsulating intermediate layer 10. Probes l1and 12, inserted in first and second heating mantles 3, 8, respectively,control the series connection of mantles 8 and 9 with the aid of acontroller 13 and a relay 14. A heatinsulating layer 15 is mounted.Layers 7, 10 and 15 are provided with lids 70, 10a and 15a.

The device operates as follows:

Controller 13 will operate relay 14, when the temperature differencebetween probe 11 and probe 12 adopts a certain value. In the process,probes 11 and 12 are inserted in the input circuit of controller 13.Current I, which is switched by relay l4 and which passes through thewindings of mantles 8 and 9 connected in series, excites per unit ofsurface a power equal to that as in mantle 7 by applying such aconstruction that the number of windings per unit of surface ispractically equal. Accordingly the temperature in mantle 9 is alwayslower than that in mantle 8 and the fluctuations in temperature inhousing 2 resulting from switching of current I by relay 14 are absorbedfor the greater part.

Further data of the embodiment.

volume of space 1 housing 2 thickness of layer 7 We claim:

I. A furnace, comprising, a hollow metal housing for containing a samplein its cavity, a first electric heating mantle surrounding said housing,a second electric heating mantle surrounding said first mantle, aheatinsulating intermediate layer disposed between said mantles, meansfor regulating flow of heat in said layer, a third electric heatingmantle having an external heat insulating layer thereon surrounding saidsecond mantle, another heat-insulating layer disposed between saidsecond and third mantles, said second and third mantles beingelectrically connected in series, wherein the power excitable per unitof surface in said second mantle and in said third mantle issubstantially equal, means for controlling the flow of heat in saidintermediate layer, each such power being controlled by said controlmeans, and the supply voltage of said first mantle being independent ofthe supply voltage of the series connectron.

1. A furnace, comprising, a hollow metal housing for containing a samplein its cavity, a first electric heating mantle surrounding said housing,a second electric heating mantle surrounding said first mantle, aheat-insulating intermediate layer disposed between said mantles, meansfor regulating flow of heat in said layer, a third electric heatingmantle having an external heat insulating layer thereon surrounding saidsecond mantle, another heat-insulating layer disposed between saidsecond and third mantles, said second and third mantles beingelectrically connected in series, wherein the power excitable per unitof surface in said second mantle and in said third mantle issubstantially equal, means for controlling the flow of heat in saidintermediate layer, each such power being controlled by said controlmeans, and the supply voltage of said first mantle being independent ofthe supply voltage of the series connection.